The present invention relates to analogs of lipoic acid, in particular lipoic acid analogs that are positively charged.
Throughout the evolution of life on Earth, the level of oxygen in the atmosphere increased. As a result, aerobic organisms had to develop efficient defense mechanisms in order to cope with increasing oxidative stress. The toxicity of oxygen is known to be due to the formation of reactive oxygen species (ROS) during the normal metabolism of a living organism. ROS include oxygen-derived free radicals and non-radical derivatives that are capable of inciting oxidative damage to biological structures. ROS have also been shown to be involved in more than one hundred different pathological syndromes and in the aging process.
There are several lines of defense against oxidative stress, including (i) macromolecules, such as enzymes, that can interact with ROS directly and remove them, or chelate metals and prevent the augmentation of oxidative damage; (ii) low molecular weight antioxidants that can interact directly with ROS, including both synthetic antioxidants and antioxidants from natural sources; and (iii) damage repair mechanisms.
In the last five years, in addition to the conventional idea that antioxidants interact with oxidants to minimize oxidative damage, a new and exciting role of these reduction-oxidation (redox) sensitive molecules (both oxidants and anxioxidants) has become clear. These molecules function in a ubiquitous redox regulation of key biological processes such as immune response, cell-cell adhesion (e.g., atherosclerosis), cell proliferation, inflammation, metabolism, glucose uptake (diabetes) and programmed cell death (apoptosis). The basic regulation mechanism is due to the existence of redox-regulated amino acids in proteins, e.g., cysteine, tyrosine and methionine. Modifications of these amino acids by either oxidants or reducing agents can trigger or inhibit the biological function of a protein.
One of the most potent naturally occurring antioxidants known is lipoic acid (LA). xcex1-Lipoic acid is also known as thioctic acid, 1,2-dithiolane-3-pentanoic acid, 1,2-dithiolane-3valeric acid or 6,8-thioctic acid. xcex1-Lipoic acid has a chiral carbon atom and occurs in two enantiomeric forms.
Biologically, lipoate exists as lipoamide in at least five proteins where it is covalently linked to a lysyl residue. Four of these proteins are found in xcex1-ketoacid dehydrogenase complexes, the pyruvate dehydrogenase complex, the branched chain keto-acid dehydrogenase complex and the xcex1-ketoglutarate dehydrogenase complex. Three lipoamide-containing proteins are present in the E2 enzyme dihydrolipoyl acyltransferase, which is different in each of the complexes and specific for the substrate of the complex. One lipoyl residue is found in protein X, which is the same in each complex. The fifty lipoamide residue is present in the glycine cleavage system.
Recently lipoic acid has been detected in the form of lipoyllysine in various natural sources. In the plant material studied, lipoyllysine content was highest in spinach (3.15 xcexcg/g dry weight; 92.51 xcexcg/mg protein). When expressed as weight per dry weight of lyophilized vegetables, the abundance of naturally existing lipoate in spinach was over three- and five-fold higher than that in broccoli and tomato, respectively. Lower concentrations of lipoyllysine were also detected in garden pea, Brussels sprouts and rice bran. Lipoyllysine concentration was below detection limits in acetone powders of banana, orange peel, soybean and horseradish, however.
In animal tissues, the abundance of lipoyllysine in bovine acetone powders can be represented in the following order: kidney greater than heart greater than liver greater than spleen greater than brain greater than pancreas greater than lung. The concentration of lipoyllysine in bovine kidney and heart were 2.64xc2x11.23 and 1.51xc2x10.75 xcexcg/g dry weight, respectively.
Lipoate in its reduced form as dihydrolipoate (DHLA) possesses two xe2x80x94SH groups which provide a very low oxidation potential to the molecule (xe2x88x920.29 V). Thus, lipoic acid and the DHLA redox couple are excellent antioxidants capable of interacting with almost all forms of ROS, recycling other antioxidants and in addition reducing oxidized disulfide groups in biological systems. These molecules then may recuperate their biological reducing power and function. All of these qualities of LA make it also one of the most important molecules in redox signaling. A good example of this is the ability of this metabolically active compound to increase glucose uptake in an insulin mimic effect.
Various of the enantiomeric forms of xcex1-lipoic acid, and combinations and derivatives thereof (including its reduced form), have been used to treat numerous conditions. For example, U.S. Pat. Nos. 5,650,429 and 5,532,269 disclose the use of lipoic acids in the treatment of circulatory disorders. U.S. Pat. No. 5,621,117 teaches that the D- and L-enantiomers of xcex1-lipoic acid have different properties, with the D-enantiomer being primarily antiphlogistic and the L-enantiomer being mainly antinociceptive (analgesic). U.S. Pat. No. 5,669,670 discloses combinations of lipoic acids and vitamins in compositions useful for producing analgesic, anti-inflammatory, antinecrotic, anti-diabetic and other therapeutic effects. U.S. Pat. No. 5,334,612 describes certain alkylated derivatives of lipoic acid and their use in treatment of retroviral diseases. U.S. Pat. No. 5,084,481 discloses the use of reduced lipoic acid (DHLA) and salts thereof in treating inflammatory diseases. U.S. Pat. No. 5,693,664 discloses use of LA and DHLA in the treatment of diabetes. U.S. Pat. No. 5,508,275 discloses a variety of lipid-selective antioxidants, including lipoic acid derivatives.
ROS are also known to be capable of activating NF-kappa B, and it is believed that ROS are the final common signal for a number of stimuli that activate NF-kappa B. Sen and Packer, The FASEB Journal, Vol. 10, 709-720 (1996). The activation of NF-kappa B is believed to be involved, at least in part, in the causation or progression of a number of disease states. Packer et al., Advances in Pharmacology, Vol. 38, 79-101 (1997). The administration of antioxidant compositions including tocotrienyl lipoates has been proposed for regulating NF-kappa B activation in U.S. Provisional Patent Application Serial No. 60/055,433, filed Aug. 4, 1997, which is incorporated herein in its entirety by reference.
Lipoic acid suffers from certain disadvantages, however. In particular, LA is reduced to DHLA within cells and then rapidly effluxed.
A need exists for an improved lipoic acid analog.
In addition to reactive oxygen species (ROS), reactive nitrogen species (RNS) such as nitrogen monoxide and byproducts thereof, in particular free radical byproducts thereof, have been implicated in inflammatory conditions such as diabetic neuropathy. Compositions which include lipoate derivatives, in particular tocotrienyl lipoates, have also been proposed for use in treating conditions in which RNS are involved, for example in U.S. Provisional Application Serial No. 60/055,433.
A need exists for improved compounds that are effective in treating conditions in which RNS are involved.
In accordance with one aspect of the present invention, there is provided a compound having the formula I: 
wherein
R1 and R2 independently denote a methylene, ethylene or unbranched or branched C3-16 alkylene, alkenylene or alkynylene group which is unsubstituted or substituted with one or more halogen, hydroxyl or amine groups, wherein in said unbranched or branched C3-16 alkylene, alkenylene or alkynylene group an internal alkylene carbon atom in the carbon backbone thereof can be replaced by an oxygen atom,
R3 and R4 
(i) independently denote
(a) hydrogen,
(b) a methyl, ethyl, vinyl or unbranched or branched C3-16 alkyl, alkenyl or alkynyl group which is unsubstituted or substituted with one or more halogen, hydroxyl or amine groups, wherein in said unbranched or branched C3-16 alkyl, alkenyl or alkynyl group an internal alkylene carbon atom in the carbon backbone thereof can be replaced by an oxygen atom,
(c) a cycloalkyl, alkylcycloalkyl, alkenylcycloalkyl or alkynylcycloalkyl group having 5 to 16 carbon atoms which is unsubstituted or substituted with one or more halogen, hydroxyl or amine groups, or
(d) an aryl, alkaryl, aralkyl, alkenylaryl, aralkenyl, alkynylaryl or aralkynyl group having 6 to 16 carbon atoms which is unsubstituted or substituted with one or more halogen, hydroxyl or amine groups, or
(ii) jointly with the nitrogen atom form a cyclic or aromatic amine which is unsubstituted or substituted with one or more alkyl, alkenyl, alkynyl, halogen, hydroxyl or amine groups,
X denotes O, S, xe2x80x94NHxe2x80x94 or xe2x80x94NR5xe2x80x94, and
R5 denotes methyl, ethyl, or unbranched or branched C3-16 alkyl,
or a pharmaceutically acceptable salt thereof, wherein said compound is in equilibrium with a protonated form thereof.
In formula I, both the R and the S enantiomer are provided.
In a preferred embodiment, the compound is in equilibrium with a protonated form thereof at physiological pH.
In a preferred embodiment, X denotes xe2x80x94NHxe2x80x94. More particularly, R1 is xe2x80x94(CH2)4xe2x80x94, R2 is xe2x80x94(CH2)2xe2x80x94, and R3 and R4 are methyl groups or hydrogen atoms. The former particularly preferred embodiment is referred to herein as xe2x80x9cLA-PLUSxe2x80x9d.
In another preferred embodiment, X denotes xe2x80x94NHxe2x80x94, R1 is xe2x80x94(CH2)4xe2x80x94, R2 is xe2x80x94(CH2)2xe2x80x94, and R3 and R4 form an unsubstituted or substituted pyridine or imidazole group together with the nitrogen atom.
In accordance with another aspect of the present invention, there is provided a compound having the formula II: 
wherein
L1 and L2 independently denote (i) a methylene group or a C6-10 arylene group which is unsubstituted or substituted with a halogen, hydroxyl, amine or unbranched or branched C3-16 alkyl, alkenyl or alkynyl group or (ii) a linking group having a carbon backbone that includes 2 to 16 carbon atoms, wherein a carbon atom in said carbon backbone can be replaced by an oxygen atom, an unsubstituted or substituted amine group, a sulfur atom, an unsubstituted or substituted C6-10 aryl group or a combination thereof,
Y denotes an ester, thioester, urethane or unsubstituted or alkyl-substituted amide linkage, and
A denotes a group containing a nitrogen atom that is in equilibrium with a protonated form thereof,
or a pharmaceutically acceptable salt thereof. Both the R and the S enantiomers are provided.
In accordance with a further aspect of the present invention, there is provided a compound having the formula III: 
wherein
W1, W2 (i) jointly denote xe2x80x94Sxe2x80x94Sxe2x80x94, xe2x80x94S(O)xe2x80x94Sxe2x80x94 or xe2x80x94Sxe2x80x94S(O)xe2x80x94, or (ii) individually denote a group including an oxidized sulfur atom,
L3 denotes (i) a single bond, (ii) a methylene group or a C6-10 arylene group which is unsubstituted or substituted with a halogen, hydroxyl, amine or unbranched or branched C3-16 alkyl, alkenyl or alkynyl group or (iii) a linking group having a carbon backbone that includes 2 to 16 carbon atoms, wherein a carbon atom in said carbon backbone can be replaced by an oxygen atom, an unsubstituted or substituted amine group, a sulfur atom, an unsubstituted or substituted C6-10 aryl group or a combination thereof,
Y denotes an ester, thioester, urethane or unsubstituted or alkyl-substituted amide linkage,
n denotes 0 or 1, and
Z denotes a group containing a nitrogen atom that is in equilibrium with a protonated form thereof and increases the intracellular retention time of said compound with respect to the analogous compound having a carboxyl group,
or a pharmaceutically acceptable salt thereof. Both the R and the S enantiomers are provided.
In accordance with another aspect of the present invention, methods of making the inventive compounds are provided. In a first embodiment, a method of making a compound having the formula I includes the step of reacting a compound having the formula IV 
with a compound having the formula V
R6xe2x80x94R2xe2x80x94NR3R4 xe2x80x83xe2x80x83V 
wherein
R6 denotes OH, SH, NH2 or NHR5.
In a second embodiment, a method of making a compound having the formula II includes the step of reacting a compound having the formula VI 
with a compound having the formula VII
Y2-L2-A xe2x80x83xe2x80x83VII 
wherein
Y1 and Y2 denote groups which react to form an ester, thioester, urethane or unsubstituted or substituted amide linkage.
In a third embodiment, a method of making a compound having the formula III includes the step of reacting a compound having the formula VI 
with a compound having the formula VIII
Y4 xe2x80x83xe2x80x83VIII 
to form a product,
wherein
Y3 denotes Y1 or a group that reacts with Y4 to form an imine group or a guanidine group,
Y4 denotes Y2xe2x80x94Z or a reagent that reacts with Y3 to form an imine group or a guanidine group, and
Y1 and Y2 denote groups which react to form an ester, thioester, urethane or unsubstituted or substituted amide linkage,
and then optionally oxidizing the product so formed.
In accordance with still another aspect of the present invention, compositions including the inventive compounds are provided. The inventive compositions include pharmaceutical compositions as well as cosmetic preparations.
In accordance with a further aspect of the present invention, methods of treating conditions in a warm-blooded animal that involve a reactive oxygen species or a redox mechanism are provided. The inventive methods include the step of administering to a warm-blooded animal having the condition an effective amount of a compound as described herein.
In preferred embodiments, the condition to be treated is a pathological condition such as diabetes, atherosclerosis, an autoimmune disease, a degenerative brain disorder such as Alzheimer""s Disease, Parkinson""s Disease, Huntington""s Disease or epilepsy, a neoplastic disease including cancer, trauma resulting from injuries such as head injuries, cerebral ischemia, hepatic disorders, or AIDS. In another preferred embodiment, the condition is a clinical condition in which apoptosis or necrosis is implicated in pathogenesis.
In accordance with still another aspect of the present invention, methods of treating conditions in a warm-blooded animal that involve a reactive nitrogen species are provided. The inventive methods include the step of administering to a warm-blooded animal having the condition an effective amount of a compound as described herein.
Other objects, features and advantages of the present invention will become apparent to those skilled in the art from the following detailed description. It is to be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not limitation. Many changes and modifications within the scope of the present invention may be made without departing from the spirit thereof, and the invention includes all such modifications.